Such a steering mechanism is described, for example, in DE-OS (German Patent application laid open to inspection) 36 15 720 (U.S. Pat. No. 4,884,647 to Mimuro) which has mechanical locking when the rear steering linkage is in straight ahead steering position.
The disadvantage is that as a result of valve leakages or other sealing defects leakage of a hydrostatic system causes displacement of the hydrostatic steering power cylinder as a result of which the positions of the front and rear steering linkages are not synchronized. Thus, it is necessary by means of a manual procedure, to turn off a selector switch for the purpose of engaging the second (rear) axle or several axles. During the next following overtravelling of the now additionally connected axle, the latter was then centered and blocked in the straight ahead position. After that, it was possible to control the axle again for steering purposes.
The prior art illustrates a manually operated valve with a centering device, whereby the centering function is performed only by a biasing spring acting to lock a piston rod in centered position. The disadvantage is that only a certain spring characteristic is available and that there is always a counterpressure. Besides, the characteristic of the device is limited.